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In photography, we often hear the term "equivalent focal length". Focal length is an important indicator of the lens, we will also consider whether it is 18mm, 50mm or 200mm when we buy the lens. But in addition to the focal length, there is also an equivalent focal length, which rookies are often unable to distinguish.

What is the difference and relationship between "equivalent focal length" and "focal length"? today we will talk about it in detail.

Focal length and viewing angle focal length can affect the viewing angle, which determines the range of view, as well as the size of real objects in the camera. The relationship between them can be seen in the picture below.

With a long focal length (such as 300mm), the viewing angle is small; the viewing angle is small and only a small range of objects can be photographed; in other words, a small range of objects fill the whole picture, which produces the effect of a telescope and can naturally capture distant scenes.

When the focal length is short (such as 15mm), the viewing angle is large; if the viewing angle is large, there are more scenes that can be photographed, and each object accounts for a small proportion of the photo, and the shooting range is wider.

In addition to the focal length, the area of the sensor can also affect the viewing angle.

A lens is placed on a full-frame camera (35mm sensor) with a large viewing angle. If you put it on a half-frame camera (APS-C), although the viewing angle of the lens is still very large, the sensor of the camera becomes smaller, and the final imaging angle becomes smaller.

[shot Circle-full frame-APSC frame]

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I think of a question I saw before: why is the lens round but the picture taken is square? (and the full lens can be used on half the camera, why can't half the lens be used on the full frame? )

It can be seen that with the same focal length of the same lens, the angle of view is different on different cameras. In order to unify the standard, the concept of "equivalent focal length" was created. The equivalent focal length on a complete frame camera (35mm sensor), also known as "35mm equivalent focal length".

Conversion factor the conversion factor of a full-frame (35mm sensor) camera is 1. There is no need to change, the number of camera marks is as many as possible.

The conversion factor of Sony and Nikon's APS-C frame camera is 1.5.

Canon's APS-C frame camera is 1.6.

Panasonic, OLYMPUS 4 stroke 3 frame camera, the conversion factor is 2.

If there is a lens, the focal length marked on the lens is 50mm. When I use it on a full-frame camera, the equivalent focal length is 50 × 1 = 50mm; when used on a Canon APS-C camera, the equivalent focal length is 50 × 1.6 = 80mm.

It can also be understood that the angle of view taken by the 50mm lens on the Canon APS-C is the same as that taken by the 80mm lens in the full picture.

The effect of the equivalent focal length on the equivalent focal length is equivalent to cutting a small piece in the original picture and then filling the whole screen, so the equivalent focal length affects only the viewing angle.

Focal length can have an impact on depth of field and distortion. Look at the focal length marked on the lens, not the equivalent focal length. For example, a 50mm lens in Canon half-frame is equivalent to 80mm, but its effect on depth of field is still 50mm effect.

The focal length also affects the safety shutter (the reciprocal of the focal length), which has something to do with the viewing angle, depending on the equivalent focal length, not the focal length marked on the lens. For example, a 50mm lens in Canon half-frame is equivalent to 80mm, and its corresponding safety shutter is no longer 1max 50 seconds, but 1max 80 seconds.

This article is from the official account of Wechat: photography Research Society (ID:shoujiPhotography), by Zhang Xiaozhang.

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